The present invention relates generally to a method and apparatus for conducting drilling operations in an earth formation and, in the alternative, for gathering information on the properties or characteristics of the earth formation surrounding a wellbore. More particularly, the present invention relates to such an apparatus and method that utilizes nuclear magnetic resonance (NMR) measurements to determine, evaluate, predict, or otherwise gather certain properties of the earth formation.
In one preferred application of the invention described herein, information on the pore pressure in the formation surrounding the wellbore is derived from NMR measurements. Such pore pressure formation can play an important role in the progress of the drilling operation. For example, knowledge of the behavior of the pore pressure within the formation can help in optimizing the type and composition of the drilling fluids used (more commonly referred to as xe2x80x9cmudxe2x80x9d or xe2x80x9cmud systemxe2x80x9d), particularly the fluid density (xe2x80x9cmud weightxe2x80x9d). Specifically, it is important during the drilling operation to avoid a large pressure differential between the mud column and the formation fluids. Excessive pressure in the mud column can lead to undesirable fracturing of the formation and to substantial loss of the drilling fluid. Reduced pressure in the mud column can, on the other hand, cause formation fluid to enter and disrupt the mud system. Both scenarios can lead to even more undesirable consequences if the formation fluids reach the surface in an uncontrolled manner, commonly known as a xe2x80x9cblowoutxe2x80x9d.
Several techniques have been employed to estimate the pore pressure in the formation, but with varying degrees of success. For example, sonic and seismic measurements may be employed to deliver information on the pore pressure based on the principle that the speed of sound in a fluid increases with increasing pressure. Yet another method of estimating pore pressure is to measure the surface pump pressure and mud volume at different pressures. In any event, there has been no attempt or suggestion to use NMR measurement techniques to gather information on the pore pressure within the earth formation surrounding a wellbore.
It is known, however, that nuclear magnetic resonance (NMR) measurements taken in a wellbore can provide different types of information about a geological formation. In the past, such measurements often were made after the wellbore had been drilled. Today it is possible to log NMR measurements while drilling (i.e., logging while drilling or LWD), thus saving time and providing valuable real-time information about the earth formation as drilling progresses. For example, such information can indicate the fractional volume of pore space, the fractional volume of mobile fluid, the total porosity of the formation, permeability of the formation, etc.
Several types of commercially available logging tools are employed to perform the NMR measurements. These tools generally include one or more large permanent magnets or electromagnets for generating a static magnetic field, B0, an antenna placed proximate the formation to be analyzed, and circuitry adapted to conduct a sequence of RF power pulses through the antenna to induce an RF magnetic field, B1, in the formation. The circuitry also includes a receiver adapted to detect signals induced in the antenna as a result of the RF pulse sequence. The induced signals can then be measured and processed to provide the desired information about the properties of the formation.
Typically, NMR logging tools are tuned to detect hydrogen resonance signals (e.g., from either water or hydrocarbons) because hydrogen nuclei arc the most abundant and easily detectable. In general, measurements of NMR related phenomena of hydrogen nuclei in the earth formation are performed by allowing some time for the static magnetic field, B0, to polarize the spinning hydrogen nuclei of water and hydrocarbons in a direction substantially aligned with B0. Then angle between the nuclear magnetization and the static magnetic field, B0 can be changed by applying a sequence of RF pulses to induce the RF field B1. Commonly, the pulse sequence employed includes a first RF pulse (i.e., the excitation pulse) having a magnitude and duration selected to re-orient the nuclear magnetization by about 90 degrees from the orientation attained as a result of B0 (i.e., the initial transverse magnetization). After a selected time, a train of successive RF pulses is applied (i.e., inversion or refocusing pulses), each of which has a magnitude and a direction selected to re-orient the nuclear spin axes by about 180 degrees from their immediately previous orientations. The frequency of the RF field needed to re-orient the nuclear magnetization (i.e., the Larmor frequency) is related to the amplitude of the static magnetic field B0 by the gyromagnetic ratio xcex3, which is unique to each isotope.
Due to inhomogeneities in the magnetic field B0, the spins in the perpendicular plane (x,y-plane) typically lose their phase coherence rapidly leading to a rapid signal decay. After each of the 180 degree RF pulses the spins are reoriented in a way such that they re-gain their phase coherence leading to the re-appearance of a signal-the spin echo. Measurement of the rate at which the spin echoes decay (i.e., the rate at which the spinning nuclei lose their alignment within the transverse plane) is referred to as a relaxation, or T2 measurement. As is known in the art, the T2 measurement may be related to the chemical and physical properties of the earth formation. For example, hydrogen nuclei in viscous oils have relatively short relaxation times, whereas hydrogen nuclei in light oils have relatively long relaxation times. Similarly, hydrogen nuclei in free water typically have longer relaxation times than those in bound water (e.g., clay-bound water).
In one aspect of the invention, a method is provided for gathering information on the pore pressure in an earth formation surrounding a wellbore. The method includes the initial steps of selecting at least one suitable property (e.g., porosity, permeability, hydrogen index, drilling fluid composition, etc.) of the drilling environment (which is defined by the wellbore and the surrounding formation) and at least one NMR parameter (e.g., T2 distribution) in an NMR measurement response. A suitable property is selected for which values over a plurality of wellbore-depths can be correlated with the characteristics or behavior of the pore pressure in the earth formation. The method further includes conducting an NMR measurement at a plurality of wellbore depths, thereby generating an NMR response from the drilling environment. The measured values of the NMR parameter in the NMR response are then correlated with values of the suitable property. Next, the values of the suitable property are compared over the plurality of depths, and then the correspondence between the property values is correlated with the behavior of the pore pressure over the plurality of depths. In this way, the characteristics of the pore pressure in the earth formation over the plurality of wellbore depths are determined. In a variation of the inventive method, the values of the selected NMR parameter over a plurality of depths are also correlated with values of the suitable property (to first determine the behavior of the suitable property) and then the behavior of the suitable property is correlated with the behavior of the pore pressure.
In another aspect of the invention, the inventive method includes the initial steps of selecting at least one suitable property of the drilling environment, whereby variations in the suitable property over a wellbore depth interval can be correlated with variations in the pore pressure in the earth formation, and predicting a profile of the suitable property over a wellbore depth interval (e.g., through historical information or preliminary measurements). Further, at least one NMR parameter is selected, such that values of the NMR parameter over the depth interval can be correlated with values of the suitable property over the depth interval. After providing an NMR measurement apparatus, the drilling operation may then commence so as to initiate forming of the wellbore.
During drilling, the NMR measurement apparatus is operated at a depth in the wellbore to generate an NMR response from the drilling environment and to account for the NMR parameter in the NMR response. By repeating this procedure at a plurality of wellbore depths and providing values of the NMR parameter at these depths, an actual profile of the suitable property is established. Deviations of the actual profile from the predicted profile may then be correlated with variations in the pore pressure in the earth formation.
In the above method, the suitable property selected may be, among other things, porosity, permeability, hydrogen index, a drilling fluid property such as composition, a formation fluid property such as composition, or combinations of these. In one specific application, the suitable property selected is depth of fluid invasion, and the NMR measurements are directed to a near-wellbore region of the drilling environment.
In yet another aspect of the invention, the invention is directed to a method of drilling a wellbore in an earth formation. The method includes the steps of commencing drilling of the wellbore in the earth formation, using drilling fluid having a fluid composition, and while drilling, monitoring the pore pressure in the earth formation surrounding the wellbore. The monitoring step further involves selecting at least one suitable property of the drilling environment such that variations in the suitable property over wellbore depths can be correlated with variations in pore pressure in the earth formation. Then, NMR measurements are obtained at a plurality of wellbore depths, thereby generating an NMR response from the surrounding wellbore over the plurality of wellbore depths. From the NMR response, the behavior of the suitable property over the wellbore depths is determined and then, the behavior of the suitable property over the wellbore depths is correlated with the behavior of the pore pressure in the earth
Other aspects of the invention are described in the Detailed Description, or specified in the appended claims. For example, the invention is also directed to a system and a tangible medium suitable for use, or at least associated with, the methods described above.